The present invention is directed in general to a water treatment method for removing arsenic from arsenic contaminated water. It is directed in particular to a treatment method wherein arsenic is removed from the contaminated water by co-precipitation with iron.
Water contaminated with arsenic may be encountered in effluent from mining and mineral processing activities, in waste ash from coal fired power plants, in wells in the vicinity of such operations, and in naturally occurring arsenical mineralization. In order to protect the environment against such arsenic contaminated water it is necessary to select arsenic removal treatment methods for effluent from activities producing the contaminated water before it can find its way into groundwater, wells or other water supplies. It is also necessary to select arsenic removal treatment methods for arsenic contaminated water which has already found its way into groundwater, wells or other water supplies.
Removal of arsenic from contaminated water by the methods including addition of an iron salt to the water are not unknown in the prior art. In such methods it is a goal to co-precipitate the iron with arsenic as insoluble precipitates and separate the precipitates from the water. Success of such methods, however, is strongly dependent on the condition of the contaminated water before it is treated. In most cases, the success of a particular method will be adversely influenced if the contaminated water to be treated has been exposed to air. For example, whatever the original state of arsenic in the arsenic contaminated water, through exposure to air it will usually have been converted to arsenious oxide (AS.sub.2 O.sub.3) or to the arsenite ion H.sub.2 AsO.sub.3 -. In most cases, these would have precipitated as arsenious oxide or as an arsenite of another element, commonly iron or calcium. In a static reaction tank, or in unfiltered water, the arsenite precipitate would be present in the water to be treated.
While the addition of iron, or an oxidant such as chlorine may cause precipitation of an insoluble arsenate for any arsenic ions remaining in solution in the water to be treated, the arsenite precipitates, being partially soluble, would cause arsenic to leach back into the water being treated. This leaching would, at best, unacceptably prolong the arsenic removal process, or leave a less than safe level of arsenic in the treated water. At worst, it may result in sufficient iron depletion in the water being treated to stop the arsenic removal process altogether.
There is clearly a need for an improved arsenic removal treatment method which may be used with arsenic contaminated water from most common sources.